1. Field of the Invention
The present invention relates to an electronic device with a cavity, of which the internal pressure is kept low, and a method for fabricating such an electronic device. More particularly, the present invention relates to an electronic device such as an infrared sensor, of which the sensing element is encapsulated within a reduced pressure atmosphere and which can further reduce the pressure, if necessary, by measuring the pressure of the atmosphere within its cavity, and also relates to a method for fabricating such an electronic device.
2. Description of the Related Art
In the prior art, an electronic device such as an infrared sensor has at least the sensing element thereof arranged in a cavity on a substrate and is encapsulated in a vacuum or inert atmosphere by a cap member so as to increase its responsivity. Examples of electronic devices of this type include not just infrared sensors but also pressure sensors, acceleration sensors, flow velocity sensors, and vacuum transistors.
Among these sensors of various types, the infrared sensors are roughly classifiable into thermo sensors including bolometer sensors, pyroelectric sensors, thermopile sensors and thermocouple sensors and quantum sensors made of PbS, InSb or HgCdTe. Most of the bolometer sensors include a sensing element made of a variable resistance material such as polysilicon, Ti, TiON or VOx. However, some bolometer sensors use the forward current transition characteristic of a PN diode, for example. The thermopile sensors may use the Seebeck effect produced at a PN junction, for example. And the pyroelectric sensors utilize the pyroelectric effect of a material such as PZT, BST, ZnO or PbTiO3. Also, a quantum sensor detects current that flows due to the excitation of electrons. Another infrared sensor uses a Chromel-Alumel Thermocouple that senses infrared radiations by utilizing the Seebeck effect.
To maintain high infrared sensitivity and sensing accuracy, the quantity of heat dissipated from the infrared sensing element thereof is preferably reduced. And it is known that if the sensing element is encapsulated in a vacuum atmosphere or reduced-pressure inert atmosphere, which is created by a micro vacuum package, for example, the sensing performance improves.
The sensitivities of pressure sensors and acceleration sensors also increase as the viscous drag of the air surrounding the sensing element decreases. That is why the sensing element thereof is also preferably encapsulated in a vacuum atmosphere or reduced-pressure inert atmosphere created by a cap member, for example. Also, if a vacuum has been created inside of a cap member, the degree of the vacuum inside the cap member is preferably checkable while the electronic device is being manufactured or being operated.
Hereinafter, a method for fabricating a conventional electronic device will be described with reference to FIGS. 1A through 1F.
First, in the process step shown in FIG. 1A, a silicon substrate 101, on which the sensing element 102 of an infrared sensor, for example, has been provided, is prepared. Next, a silicon dioxide film 103 is deposited on the substrate by a CVD process, for example, and then patterned so as to cover the sensing element 102 and its surrounding portion. This silicon dioxide film 103 will function as a sacrificial layer and will be etched away later to define the shape of a cavity in a subsequent process step.
Next, in the process step shown in FIG. 1B, a polysilicon film 104 is deposited by a CVD process so as to cover the silicon dioxide film 103. This polysilicon film 104 will be the sidewall and ceiling wall of a cap member for an electronic device.
Subsequently, in the process step shown in FIG. 1C, a lot of etch holes 111 are cut through the polysilicon film 104 so as to reach the silicon dioxide film 103.
Thereafter, in the process step shown in FIG. 1D, hydrofluoric acid is injected through the etch holes 111 to dissolve the silicon dioxide film 103 and then the solution is drained through the etch holes 111. As a result, a cavity 112 surrounded with the polysilicon film 104 is defined and the sensing element 102 of the sensor is exposed inside the cavity 112.
Next, in the process step shown in FIG. 1E, another polysilicon film 106 is deposited by a CVD process on the polysilicon film 104. In this process step, the polysilicon film 106 is also deposited on the inner walls of the etch holes 111, and the etch holes 111 are closed as a result. The polysilicon film 106 is further deposited on the inner walls of the cavity 112 after the CVD process has started and before the etch holes 111 are fully closed.
This CVD process is ordinarily carried out using a reaction gas such as SiH4 at a pressure of about 500 mTorr (=approximately 67 Pa). Thus, the cavity 112 is closed airtight so as to have an internal pressure of about 500 mTorr (=approximately 67 Pa) as in the CVD process. Also, as a result of this CVD process, non-reacted SiH4 gas and H2 gas, produced by the reaction, still remain in the cavity 112. In addition, the polysilicon film 106 that has been deposited on the inner walls of the cavity 112 adsorbs the non-reacted SiH4 gas and the H2 gas produced by the reaction.
Then, in the process step shown in FIG. 1F, the entire substrate 101 is heated to an elevated temperature of 500° C. or more within a high vacuum. As a result, the SiH4 gas decomposes to a certain degree inside the cavity 112 and the H2 gas is emitted through the polysilicon films 104 and 106. Consequently, the pressure inside the cavity 112 becomes somewhat lower than the internal pressure of the cavity 112 during the CVD process and the degree of vacuum of the cavity 112 increases slightly.
Such a manufacturing process is described in Japanese Patent Application Laid-Open Publication No. 2000-124469, for example.
Next, a conventional technique of increasing the degree of vacuum inside a vacuum package (i.e., cap member) and a conventional technique of measuring the degree of vacuum (i.e., pressure) will be described.
FIG. 39 schematically illustrates a cross-sectional structure of a conventional electronic device with a vacuum package (see Japanese Patent Application Laid-Open Publication No. 11-326037). The electronic device shown in FIG. 39 includes a silicon substrate 391 and a transmissive window 394 fixed on the silicon substrate 391 with solder 399. A gap 393 with a height of about 1 mm to 10 mm is provided between the transmissive window 394 and the silicon substrate 391. And a getter 395 with a size of several mm is disposed in this gap 393.
The transmissive window 394 has a number of through holes 397, through which the getter 395 has been introduced into the gap 393. When the silicon substrate 391 is placed in a vacuum, the gap 393 is evacuated through the through holes 397 to have a reduced pressure. By melting the vacuum creating solder 399, the through holes 397 are sealed up to maintain a vacuum in the gap 393. Thereafter, when the getter 393 is activated, the pressure in the gap 393 can be further reduced and a high vacuum is produced.
The degree of vacuum in the cap member may be measured with a Pirani gage, for example. A Pirani gage is an instrument for calculating the degree of vacuum based on the electrical resistance of a resistor that is placed in a vacuum. The thermal conductivity of a gas depends on the pressure (i.e., the degree of vacuum) of the gas. That is why if the thermal conductivity from a heated resistor into a gas is obtained, the degree of vacuum of the gas can be figured out by making an appropriate calibration.
Recently, as electronic devices have become smaller and smaller, there has been increasing demand for making the vacuum package (or cap member) in a very small size. For example, an image sensor, including a huge number of infrared sensing elements and visible light detecting elements that are arranged in matrix on the same substrate, was proposed. In such an image sensor, each of those infrared sensing elements with dimensions of about 50 μm square is encapsulated within a micro vacuum package with dimensions of about 100 μm square (see Japanese Patent Application Laid-Open Publication No. 2003-17672).
Meanwhile, an electronic device, including an FEA element that performs a high-speed switching operation in a vacuum and a transistor on the same substrate, is disclosed by. C. Y. Hong and A. I. Akinwande in Silicon Metal-Oxide-Semiconductor Field Effect Transistor/Field Emission Array Fabricated Using Chemical Mechanical Polishing, J. Vac. Sci. Technol. B, Vol. 21, No. 1, pp. 500 to 505, January/February 2003. To reduce the size of such an electronic device, a structure in which only the FEA element is encapsulated within a very small vacuum package is preferably adopted.
Next, another conventional electronic device, of which the infrared sensors are also encapsulated within a vacuum and which can sense a vacuum leakage, will be described with reference to FIG. 40 (see Japanese Patent Application Laid-Open Publication No. 10-224297).
The electronic device shown in. FIG. 40 includes a semiconductor substrate 411 provided on a metallic supporting member 410, and an infrared sensor 412 arranged on the surface of the semiconductor substrate 411. Another supporting member 419, which has a cavity 413 that houses the infrared sensor 412, is provided on the semiconductor substrate 411. The supporting member 419 and the semiconductor substrate 411 are fixed on the metallic supporting member 410 with epoxy resin 500.
The supporting member 419 includes an infrared input portion 418 and a diffused resistor 417. The degree of vacuum in the cavity 413 can be detected by the deformation of the diffused resistor 417.
If there is a vacuum leakage in the cavity 413 to cause a variation in internal pressure, then the infrared input portion 418 is deformed due to the pressure variation and the resistance value of the diffused resistor 417 changes as a result. Thus, by detecting a variation in current value corresponding to this variation in resistance, the vacuum leakage can be sensed.
In the electronic device manufacturing process described above, in the heat treatment process step shown in FIG. 1F, the SiH4 gas is decomposed in the cavity 112 and H2 gas is emitted out of the cavity 112. Thus, the degree of vacuum in the cavity increases slightly compared to the pressure of 500 mTorr (=approximately 67 Pa) during the CVD process. However, the degree of vacuum is preferably further increased to heighten the sensitivity of the sensor. Nevertheless, it is difficult to completely exhaust the SiH4 and H2 gases that remain inside the cavity or on the walls. That is why the increase in sensitivity is still insufficient, which is a problem.
In the manufacturing process described above, no cavity is provided between the sensing element 102 and the substrate 101. However, if sacrificial layers are provided both over and under the sensing element 102, a structure that can sense the in-cavity atmospheric gas both over and under the sensing element 102 can be obtained.
FIG. 2 is a perspective view illustrating the sensing element of a bolometer infrared sensor with such a structure and its surrounding portions. In FIG. 2, a resistor 151 called a “bolometer” and functioning as an infrared sensing element and a supporting member 152 that supports the resistor 151 thereon are provided on a substrate 101. The resistor 151 may be a patterned polysilicon film, for example, and the supporting member 152 is often a stack of a polysilicon film, a nitride film, an oxide film and other films. The supporting member 152 includes a body portion, on which the resistor 151 is provided, and arm portions extending from the body portion, and is fixed on the substrate 101 with these arm portions.
No cavity wall members are shown in FIG. 2. In an actual infrared sensor, however, the supporting member 152 is arranged within a cavity similar to the cavity 112 shown in FIG. 1F.
Hereinafter, it will be described in more detail what problems would arise if the etch holes were closed during a CVD process.
Although not shown in FIG. 2, when an infrared ray enters the resistor 151 through the polysilicon films (i.e., the films identified by the reference numerals 104 and 106 in FIG. 1F) surrounding the cavity, the temperature of the resistor 151 rises and the resistance value thereof changes with this rise in temperature. By measuring this variation in resistance value, the infrared sensor having the structure shown in FIG. 2 can detect the intensity of the infrared ray that has entered the resistor 151.
To increase the responsivity of the infrared sensor, the rise in the temperature of the resistor 151 upon the incidence of an infrared ray onto the resistor 151 needs to be increased. For that purpose, the resistor 151 functioning as an infrared sensing element is preferably thermally insulated from its external environment as fully as possible.
The conduction of heat between the resistor 151 and its external environment may be classified into conduction of heat between the resistor 151 and the substrate 101 by way of the supporting member 152 connecting them and conduction of heat from the resistor 151 through its surrounding gas.
The conduction of heat by way of the supporting member 152 decreases as the cross-sectional area of the thinnest portions of the supporting member 152 decreases and as the distance from those portions to the substrate 101 increases. According to the micro-electro-mechanical systems (MEMS) technologies, for example, portions of the supporting member 152 that are connected to the substrate 101 (i.e., the connecting portions) may be two columns of Si3N4 with a cross-sectional area of 3 μm2 and a length of 50 μm as shown in FIG. 2. In that case, the thermal conductance will be 3×10−7 W/K.
Meanwhile, the thermal conductance through the gas that surrounds the resistor 151 decreases as the pressure of the gas decreases. That is why to increase the sensitivity of the infrared sensor, the pressure of the gas surrounding the sensing element needs to be reduced.
In the conventional manufacturing process that has already been described with reference to FIGS. 1A through 1F, however, the pressure inside the cavity 112 is maintained at about 500 mTorr (=approximately 67 Pa) by the remaining gas after the process step shown in FIG. 1E has been performed. By performing a vacuum high-temperature process after the cavity 112 has been created, the internal hydrogen diffuses and leaves the cavity 112, thus decreasing the pressure inside the cavity 112 to a certain degree. However, even after having been heated to that high temperature, SiH4 gas or H2 gas still cannot be exhausted out of the cavity 112 but remains there.
In an infrared image sensor of the bolometer type, for example, its sensitivity changes with the pressure of the air surrounding the sensing element as shown by the curve in FIG. 3. Such a relationship is described in “Uncooled Infrared Imaging Arrays and Systems”, Academic Press, p. 115, for example.
In the graph of FIG. 3, the ordinate represents the sensitivity and the abscissa represents the pressure of the atmosphere in the sensing element 12. As can be seen from this graph, the lower the pressure, the higher the sensitivity. For example, the sensitivity at a pressure of 50 mTorr is about three times as high as that at a pressure of 500 mTorr. That is why the pressure inside the cavity is preferably 50 mTorr or less.
Also, the supporting member 152 of the sensing element 151 of the infrared sensor has a fine structure such as that shown in FIG. 2. For that reason, if the substrate were heated to an excessively high temperature in the process step shown in FIG. 1F, then thermal stress would be caused in, and might do some damage on, the supporting member 152.
Furthermore, if the substrate were heated to a high temperature of 660° C. or more, then Al used in the wiring of the sensor would melt. To avoid this problem, the substrate needs to be heated to less than this temperature. Meanwhile, since the outward diffusion rate of H2 is very small at this temperature, it is not possible to expect the effect of increasing the degree of vacuum significantly from this heating process.
Consequently, according to the conventional manufacturing process in which the etch holes are closed by a CVD process, it is difficult to further increase the degree of vacuum of the cavity 112 and thereby increase the responsivity.
Even if the method that has already been described with reference to FIG. 39 is adopted to increase the degree of vacuum, it is extremely difficult to arrange the getter shown in FIG. 39 in a very small cavity with good yield. As a typical conventional getter, St 171 produced by SAES Getters S. p. A., of which the headquarters are located in Italy, is known in the art. Such a getter is formed by making a getter material by sintering a powder consisting essentially of Zr and depositing the getter material on the surface of a heater. The heater usually has a wire shape and the overall thickness of the getter exceeds 1 mm.
Also, if the size of the vacuum package (cap member) is reduced to 1 mm or less, it becomes even more difficult to arrange the gettering agent in the vacuum package by the conventional method. For example, if each of a lot of infrared sensing elements is encapsulated within a micro vacuum package with dimensions of about 100 μm×100 μm, then it is very difficult and troublesome to arrange the gettering agent in one of those vacuum packages after another. In particular, in the conventional process in which a high vacuum is created inside a cap member by a getter as disclosed in Japanese Patent Application Laid-Open Publication No. 2003-17672, the getter with a size of several mm or more is fixed with solder, for example. It is not easy to carry out such a process step by a normal semiconductor silicon process, thus increasing the cost and making it impossible to apply the technique to an ultra small vacuum package.
In most of conventional degree-of-vacuum sensing techniques, the Pirani gage thereof is designed in order to measure the degree of vacuum in the vacuum chamber of a big apparatus. That is why even a relatively small Pirani gage has a sensing element with a length of about 0.2 inch. Consequently, the conventional Pirani gage is not qualified to measure the internal pressure of the ultra small vacuum package mentioned above.
Also, the method of determining the degree of vacuum as disclosed in Japanese Patent Application Laid-Open Publication No. 10-224297 is designed for a small vacuum package but uses the deformation of the infrared input portion 418 of the supporting portion 419 that defines the cavity 413. Thus, according to this technique, it is possible to determine whether or not a vacuum has been created in the cavity 413 and sense how much the degree of vacuum has increased or decreased but it is not possible to figure out the absolute value of the degree of vacuum.
To calculate the absolute value of a degree of vacuum, a method of evaluating the relationship between the degree of vacuum and the output signal in advance may be adopted. That is to say, the variation in the magnitude of deformation of the infrared input portion 418 or the variation in the resistance value of the diffused resistor 417 needs to be calculated and calibrated with respect to the degree of vacuum of the cavity 413.
If such a method were adopted for the electronic device described above, then the supporting member 417 should be subjected to a machining process of cutting a hole through it such that the cavity 413 of the electronic device shown in FIG. 40 is connected to, and evacuated by a vacuum system, and resistance value of the diffused resistor 417 should be measured while monitoring the degree of vacuum with the vacuum system.
However, it should be very difficult, and would take a lot of cost, to connect a very small cavity to a vacuum system by making such a device by machining. Thus, it is not a practical choice.
Furthermore, even if a small hole is cut through the vacuum package and then the whole device is just put into a vacuum system in order to evaluate the relationship between the degree of vacuum and the output signal in advance, a vacuum will be created not only inside the package but also outside of it. Thus, no deformation will be caused due to a pressure difference and the object described above is not achievable.
Meanwhile, it is possible to predict the magnitude of deformation of the infrared input portion 418 with respect to the absolute value of the degree of vacuum and estimate the variation in the resistance value of the diffused resistor 417 by computer simulations. In that case, however, the degree of vacuum can be calculated roughly but the absolute value thereof cannot be figured out accurately. Besides, to sense the change of the degree of vacuum accurately, the infrared input portion 418 must be further thinned and must have a greater magnitude of deformation. For that purpose, the strength of the infrared input portion 418 should be sacrificed. That is why the accuracy cannot be increased beyond a certain limit.
On top of that, to achieve an even higher degree of vacuum, the supporting member 419 and infrared input portion 418 need to have increased mechanical strengths to withstand a high vacuum. However, if the mechanical strengths are increased, then deformation is less likely to be caused and it will be more difficult to measure the degree of vacuum based on the deformation. That is why the method described above cannot cope with a high vacuum situation.
As described above, the conventional technique disclosed in Japanese Patent Application Laid-Open Publication No. 10-224297 can be used to sense an increase or decrease in the degree of vacuum in a cavity but cannot be used to figure out the absolute value of the degree of vacuum, which is a serious problem.
In order to overcome the problems described above, an object of the present invention is to provide an electronic device, which is arranged in a cavity at least partially and which can measure the pressure in the cavity, and a method for fabricating such an electronic device.
Another object of the present invention is to provide an electronic device, which can easily maintain or increase the degree of vacuum in an ultra small vacuum package, and a method for fabricating such an electronic device.